This invention relates to a control device for a direct injection engine provided with an injector which injects fuel directly into a combustion chamber.
A direct injection engine having an injector for injecting fuel directly into a combustion chamber is conventionally known. This engine is operated such that a condition in which a mixture is locally distributed around a spark plug is produced by injecting the fuel in a latter half of a compression stroke to perform so-called stratified charge combustion in a low-load state. This operation makes it possible to increase the air-fuel ratio to produce a leaner mixture while maintaining combustion stability and improve fuel economy.
Exhaust gases from engines of motor vehicles, for instance, contain hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), and there exists a demand today for reducing generation and release of these harmful constituents as much as possible to achieve improved properties of these emissions. One approach that has conventionally been taken is to provide a catalyst in an exhaust gas passage, and it is a common practice in the aforementioned direct injection engine as well to provide a catalyst in its exhaust gas passage. A generally known example of such catalyst is a three-way catalyst which has the ability to clean out HC, CO and NOx approximately at the stoichiometric air-fuel ratio. Another example that has already been developed is a catalyst which can clean out NOx even in a "lean" operating range in order to be suited to lean burn operation by stratified charge combustion in the aforementioned direct injection engine or else.
A fuel injection control device disclosed in Japanese Unexamined Patent Publication No. 4-231645, for example, is known as a device for achieving an improvement in emission converting performance of a catalyst at low temperatures, for instance, in this type of direct injection engine. In a direct injection engine having a lean NOx catalyst provided in an exhaust gas passage, the lean NOx catalyst being of a type that requires HC for the reduction of NOx, this device is so arranged as to make primary injection from an injector in a latter part of a compression stroke, and make secondary injection in addition to the aforementioned primary injection to inject a small amount of fuel for supplying HC to the lean NOx catalyst within a period from an intake stroke to an early part of the compression stroke when the temperature of the catalyst is low, or make the secondary injection in addition to the aforementioned primary injection within a period from the latter half of the combustion stroke to an early part of an exhaust stroke when the temperature of the catalyst is high. In this device, HC derived from the fuel injected by the secondary injection is supplied to the catalyst in the exhaust gas passage by setting the amount of fuel injected by the secondary injection to such a small level that will scarcely affect combustion within a combustion chamber, and a low boiling-point constituent of HC is supplied to the catalyst in low-temperature conditions while a high boiling-point constituent of HC is supplied to the catalyst in high-temperature conditions by varying the timing of the secondary injection between the low-temperature and high-temperature conditions in the aforementioned manner.
While the secondary injection is made prior to the primary injection, which is made in the latter part of the compression stroke, when the temperature of the catalyst is low in the device disclosed in the aforementioned Publication, this secondary injection is intended to supply HC to the catalyst and is made in an extremely small such that the majority of the injected fuel would reach the catalyst without burning and serve to discharge HC into the exhaust gas passage. Therefore, this device is advantageous only when a lean NOx catalyst of the type that requires HC for the reduction of NOx is used. Moreover, the device makes it possible to exhibit NOx converting effects with the supply of HC only after the catalyst has been activated to a certain degree, though it is still in a low-temperature state. Since HC is released in an earlier unheated state than that point, the device does not have the function of performing the catalyst quick light-off operation as a result of an increase in the exhaust gas temperature under such conditions.
Although it is desirable that the state of combustion be adjusted such that the combustion in the combustion chamber itself enhances HC and NOx reduction effects and quick light-off effects due to the increased exhaust gas temperature while maintaining combustion stability when the catalyst is in its unheated state, in which its temperature is lower than its activation temperature, the device of the aforementioned Patent Publication does not sufficiently provide such effects.
Furthermore, the temperature state of an engine does not necessary coincide with that of a catalyst. For example, although the catalyst rapidly cools down when the engine is stopped, the engine temperature drops slowly. Therefore, if the engine is restarted before it fully cools down after it was once stopped, for instance, the engine may reach its heated condition, in which its temperature is higher than a specified temperature, when the catalyst is still in its unheated state. Fuel evaporation and atomization conditions within the combustion chamber could change if the engine temperature status varies when the catalyst is still in its unheated state. Although such changes in the fuel evaporation and atomization conditions could affect the combustion stability and quick light-off effects, for instance, due consideration has not been given to such points in the prior art.